stepper acceleration rate - limits?

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I haven't got a detailed torque curve for the 12Nm motor, this is the only thing I have:



but it's for a 100VAC driver and that particular motor is a 4 wire version, I'm using an 8 wire connected in parallel. I've asked Leadshine for a torque curve for their own 12Nm just to get an idea of what can be expected and it looks like this:



As you can see, it also drops off pretty quickly for the VDC EM806 drive, performs much better with VAC and higher voltages. Looking at the speed values and the EM806 at 72VDC, the drop off point is around 80-100RPM, afterwards torque falls dramatically, with only around 3Nm available at 240RPM, this is much worse than the less powerful 8Nm motor I was using before which has a torque curve like this:

I wouldn't mind using the 8Nm instead of the 12Nm, but unfortunatelly I need the start torque to be at around 12-10Nm, 7Nm from the smaller motor isn't acceptable and causing stalling pretty much on startup.

To answer some questions:

Jonathan: the 12Nm is resonant but at around 80RPM, I haven't noted down the exact speed, but I can see / hear the whole motor / belt vibrating pretty badly, but it's only a short period, once it's past that speed stage the vibration is gone. When I say stalling it's really loosing steps and hardly rotating, a lot of jiggery action going on with the motor pretty much at stand still. With the HBS86 driver that would result in a Stall Fault and stopping the motor, on the EM806 it still wants to move loosing majority of the steps, but the stall alarm doesn't go off (it's got sensorless stall detection, not sure how that works). I simply stop the motor manually at that stage as I can see it's not going to go anywhere.

irving: it's 68VDC on the EM806 driver, here's the inductance data, the 12Nm is used in parallel:



This is the connections on the driver:

Phase A: RED + BLU
Phase A': YEL + BLK
Phase B: WHT + BRN
Phase B': ORG + GRN

Regards,
T.